Self-adaptive internal and external double-circulation thermal management system for transformer and control method thereof

Abstract

The invention discloses a self-adaptive internal and external double-circulation thermal management system for a transformer and a control method thereof. The self-adaptive internal and external double-circulation thermal management system comprises a transformer main body, an external circulation cooling device, an internal circulation assembly, an automatic control valve group and a heat exchange unit. The heat exchange units are fixed in annular gaps between the high-voltage coil and the low-voltage coil at equal intervals, the honeycomb cavities filled with the phase-change materials are heated to expand and then extrude and seal the heat conduction pipes through the elastic films, generated hydraulic pressure is transmitted to oil return cavities of the self-control valve sets through the adjusting pipes, valve elements are pushed to move upwards, the overflowing opening degree is increased, and therefore the outer circulation cooling flow is increased in a self-adaptive mode. A shape memory alloy spring is arranged at the upper end of the self-control valve set, the elastic pressure of the spring is intelligently adjusted by a controller according to a sensor signal, and the valve opening threshold value is dynamically corrected. According to the invention, two-stage cooperative thermal management with physical self-driving as a main part and intelligent correction as an auxiliary part is realized, the dual advantages of intrinsic safety and accurate control are realized, and the short-time overload capacity of the transformer is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of thermal management technology for power transformers, and more specifically, to a cooling system and control method for an oil-immersed transformer. Background Technology

[0002] Oil-immersed transformers use insulating oil as a cooling medium to dissipate the heat generated by the core and windings during operation through heat sinks on the tank walls. This maintains the winding hot spot temperature within the tolerance range of the insulation material, ensuring safe operation and extending the transformer's lifespan. Existing cooling methods for oil-immersed transformers are mainly divided into two categories. The first is natural oil circulation cooling (ONAN), which utilizes the principle that transformer oil naturally rises as its density decreases when heated and sinks when cooled, forming an internal convection circulation. This method is simple in structure and highly reliable, but its thermal response is slow; oil temperature changes lag behind winding hot spot temperatures by 10-30 minutes. When the transformer encounters a short-term overload (such as peak load at a charging station or the startup impact of a data center server), the winding temperature may spike instantly before the cooling system responds, leading to accelerated insulation aging or even breakdown. The second type is forced oil circulation cooling (OFAF / ODAF), which uses an oil pump to force the oil to flow and combines this with a fan to enhance heat dissipation. While such solutions improve cooling efficiency to some extent, they generally adopt a single-pump global oil supply mode, which cannot accurately control the thermal imbalance of the three-phase windings by zone. At the same time, their control logic is still based on temperature threshold triggering, which is essentially "responsive" rather than "predictive" thermal management, making it difficult to fundamentally overcome the thermal inertia problem.

[0003] To address short-term overloads, recent research has proposed filling transformer oil channels with phase change materials (PCMs) to absorb peak heat through their solid-liquid phase change. However, this approach suffers from several key drawbacks: First, PCM passively absorbs heat, making the phase change process uncontrollable and unable to be actively linked with the cooling system. Second, the phase change state (solid-liquid ratio) of the PCM cannot be monitored in real time, preventing the control system from quantifying the remaining heat storage capacity. Without additional cooling enhancement after the PCM has completely melted, heat will rapidly accumulate. Third, the PCM experiences an "isothermal plateau" during phase change, where the temperature remains relatively constant while the phase change continues. Relying solely on temperature sensors cannot determine the actual melting progress, leading to blind spots in the control strategy. To improve thermal management accuracy, some improvements have introduced sensor networks and edge computing nodes, employing complex algorithms for predictive control. However, under the harsh conditions of high temperatures and strong electromagnetic fields in transformers, these solutions frequently encounter problems such as sensor lead insulation failure and electronic component malfunctions, becoming new sources of failure and creating the paradox of "introducing unreliable factors to improve reliability."

[0004] In summary, existing transformer thermal management technologies present an irreconcilable contradiction between rapid response, precise control, and system reliability. How to achieve rapid adaptive cooling of local hot spots (between high and low voltage coils) while retaining the high reliability of natural convection cooling, and how to solve fundamental problems such as the unknowable phase transition state of the PCM and control lag, have become urgent technical bottlenecks to be overcome in this field. Summary of the Invention

[0005] To address the shortcomings of existing transformer thermal management technologies, such as response lag, this invention provides a transformer adaptive internal and external dual-circulation thermal management system and its control method. By constructing a two-level collaborative mechanism with physical self-driving as the main component and intelligent correction as the auxiliary component, it achieves rapid adaptive cooling of local hot spots in the coil and solves the problem of deep coupling between phase change material state sensing and flow regulation.

[0006] The solution to the technical problem of this invention is as follows: A transformer adaptive internal and external dual-circulation thermal management system is adopted, including a transformer body. The transformer body has an oil cavity, within which is an iron core, a low-voltage coil mounted on the outside of the iron core, and a high-voltage coil mounted on the outside of the low-voltage coil. An annular gap is provided between the low-voltage coil and the high-voltage coil. The oil cavity has an existing cooling channel that relies on natural convection of the oil. The system also includes: an external circulation cooling device, located outside the transformer body, for forming an independent external oil circulation cooling circuit; and an internal circulation component, located inside the oil cavity, including an inlet pipe connected to the external circulation cooling device, a collector pipe, and multiple guide pipes connecting the inlet pipe and the collector pipe. A heat pipe; multiple phase change heat transfer modules disposed in the annular gap between the low-voltage coil and the high-voltage coil, each phase change heat transfer module including a heat-conducting shell and a phase change material filled in the heat-conducting shell, the heat pipe penetrating the interior of the heat-conducting shell and adjacent to the phase change material; a flow regulation unit disposed in the external circulation cooling circuit for regulating the circulation flow rate of the external circulation cooling device; a pressure transmission unit connected between the phase change heat transfer modules and the flow regulation unit for transmitting the pressure change caused by the thermal expansion of the phase change material to the flow regulation unit; wherein, when the phase change material expands due to heat, the pressure transmission unit drives the flow regulation unit to increase the circulation flow rate of the external circulation cooling device.

[0007] A transformer thermal management control method based on the system is adopted, comprising the following steps: Step S1: Under normal load conditions, the oil is cooled by vertical convection cooling through the original natural convection cooling channels within the transformer body; Step S2: When the load increases, causing the temperature between the low-voltage coil and the high-voltage coil to rise, the phase change material in the phase change heat exchange module absorbs heat and expands. The pressure change generated by the expansion is transmitted to the flow regulation unit through the pressure transmission unit, driving the flow regulation unit to increase the circulation flow of the external circulation cooling device, so that the cooling oil flows through the heat-conducting pipe of the internal circulation component through the phase change heat exchange module for enhanced heat exchange; Step S3: When the load returns to normal and the coil temperature drops, the phase change material solidifies and contracts, the pressure of the pressure transmission unit decreases, the flow regulation unit resets, the circulation flow of the external circulation cooling device decreases, and the system returns to step S1.

[0008] The beneficial effects of this invention are as follows: 1. By directly driving the self-controlled valve assembly with hydraulic pressure generated by the expansion of the phase change material within the heat exchange unit, the thermal response time is shortened from the existing 10-30 minutes lag to within seconds. Tests in Example 1 show that under a 180% load impact, the system response delay is <1 minute, achieving true synchronization of temperature rise and enhanced cooling.

[0009] 2. Multiple heat exchange units operate independently, each corresponding to an automatic control valve group, which can autonomously adjust the local cooling flow rate according to the actual heat load of each area. Compared with the traditional single-pump global oil supply scheme, this system avoids the normal energy waste caused by global over-design. This system does not replace or affect the original natural convection cooling channels of the transformer; the two operate in parallel and are mutually redundant. Even in the event of intelligent correction or external circulation device failure, the basic cooling capacity is still maintained.

[0010] 3. This system is particularly suitable for scenarios with drastic load fluctuations, high short-term overload requirements, and stringent reliability requirements, including distribution transformers for new energy vehicle charging stations, dedicated transformers for data centers, traction rectifier transformers for rail transit, grid-connected transformers for new energy power plants, and unattended outdoor substations. Its "physical self-driving + intelligent correction" two-level collaborative architecture provides a reliable foundation for subsequent more advanced thermal management algorithms (such as predictive control based on digital twins and load pre-adaptation linked with grid dispatch). Reserved communication interfaces and modular design facilitate integration with existing substation monitoring systems. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the overall structure of the system of the present invention; Figure 2 yes Figure 1 Front view and AA section view; Figure 3 yes Figure 1 Internal structure diagram; Figure 4 This is a schematic diagram of the piping layout for the internal circulation component; Figure 5 This is a structural cross-sectional view of the heat exchange unit and the automatic control valve assembly; Figure 6 This is a schematic diagram of the external circulation cooling device; Figure 7 This is a schematic diagram showing the layout relationship between the heat exchange unit and the high and low voltage coils; Figure 8 This is a partially enlarged schematic diagram of the heat exchange unit; Figure 9 This is a flowchart illustrating the control method steps of the present invention.

[0012] Numbering in the diagram: 1-Transformer body; 2-External circulation cooling device; 3-Internal circulation assembly; 4-Automatic control valve group; 5-Heat exchange unit; 11-Oil chamber; 12-Front and rear walls; 13-Side walls; 14-Fixed heat sink; 15-Traveling frame; 16-Iron core; 17-Low-voltage coil; 18-High-voltage coil; 21-External oil tank; 22-External oil tank inlet pipe; 23-External oil tank outlet pipe; 24-Constant pressure tank; 25-Return pipe; 26-Independent heat sink ; 27-Constant pressure pump; 31-Multi-port pipe; 32-Branch pipe; 33-Inlet pipe; 34-Lower ring pipe; 35-Heat conduction pipe; 35a-Closed heat conduction pipe; 36-Upper ring pipe; 37-Outlet pipe; 38-Regulating pipe; 41-Valve body; 42-Valve inlet; 43-Valve outlet; 44-Valve core; 45-Flat orifice; 46-Oil return chamber; 47-SMA micro-valve; 51-Heat conduction square tube; 52-Honeycomb cavity; 53-Elastic membrane; 6-Convection cavity. Detailed Implementation

[0013] The present invention will now be described in further detail with reference to the accompanying drawings. It should be noted that the drawings are simplified schematic diagrams, intended to clearly illustrate the structural relationships related to the innovative points of the technical solution, and are not intended to limit the actual product size, proportions, or complete form. The same numbers in the drawings represent identical or functionally equivalent components.

[0014] Example 1: The overall structure of the system is as follows Figure 1 , Figure 2 and Figure 3As shown, the transformer adaptive internal and external dual-circulation thermal management system of the present invention includes a transformer body 1, an external circulation cooling device 2, an internal circulation component 3, a self-controlled valve group 4, and a heat exchange unit 5. The transformer body 1 adopts a conventional oil-immersed transformer structure, with a closed oil cavity 11 inside. The oil cavity 11 contains an iron core 16, a low-voltage coil 17, and a high-voltage coil 18. The low-voltage coil 17 is fitted outside the iron core 16, and the high-voltage coil 18 is fitted outside the low-voltage coil 17. An annular gap is maintained between the low-voltage coil 17 and the high-voltage coil 18. This gap is an original structure from the transformer manufacturing process and is used for oil flow and heat dissipation. Within the oil cavity 11, natural vertical convection is formed by the density change of the oil after heating; this is the original cooling channel 6 (i.e., the natural convection channel). Fixed heat sinks 14 are provided on the front and rear walls 12 of the transformer body 1 for exchanging heat from the oil cavity 11 to the outside. A traveling frame 15 is installed at the bottom of the transformer body 1 for easy equipment movement.

[0015] An external circulation cooling device 2 is located outside the side wall 13 of the transformer body 1, forming an external oil circulation cooling circuit independent of the original natural convection system. An internal circulation assembly 3 is located inside the oil chamber 11 and communicates with the external circulation cooling device 2. Multiple heat exchange units 5 are fixed at equal intervals within the annular gap between the low-voltage coil 17 and the high-voltage coil 18. An automatic control valve assembly 4 is installed inside the side wall 13, used to automatically adjust the flow rate of the external circulation cooling circuit according to the temperature status of the heat exchange units 5.

[0016] External circulation cooling device such as Figure 6 As shown, the external circulation cooling device 2 includes an external oil tank 21, independent heat sinks 26, a constant pressure tank 24, and a constant pressure pump 27. The external oil tank 21 is a flat plate-type box structure, welded from aluminum alloy or stainless steel, and fixed to the outside of the side wall 13 of the transformer body 1, tightly fitting against the side wall 13 to utilize some of the heat conducted by the side wall. Independent heat sinks 26 are welded to the outer surface of the external oil tank 21. The heat sinks 26 have a finned structure with a fin spacing of 8-12mm and a height of 40-60mm, used to dissipate the heat of the oil inside the external oil tank 21 into the surrounding air.

[0017] The upper end of the outer oil tank 21 is connected to the outer oil tank inlet pipe 22. The outer end of the outer oil tank inlet pipe 22 communicates with the outer oil tank 21, and its inner end passes through the side wall 13 and connects to the outlet pipe 37 of the inner circulation component 3. The lower end of the outer oil tank 21 is connected to the outer oil tank outlet pipe 23. The inner end of the outer oil tank outlet pipe 23 communicates with the outer oil tank 21, and its outer end is connected to the oil inlet of the constant pressure tank 24 through a pipeline. The constant pressure tank 24 is a closed pressure vessel with a diaphragm or piston structure inside to isolate the oil from the gas. A constant pressure pump 27 is installed on the constant pressure tank 24. The constant pressure pump 27 and the pressure sensor inside the tank form a closed-loop control circuit. When the pressure inside the tank is lower than the set value (e.g., 0.1 MPa), the constant pressure pump 27 starts to replenish the pressure, so that the outlet of the constant pressure tank 24 always maintains a constant pressure. The oil outlet of the constant pressure tank 24 is connected to the return pipe 25, and the other end of the return pipe 25 passes through the side wall 13 and connects to the valve inlet 42 of the automatic control valve group 4. Manual ball valves or solenoid valves can be installed on the external oil tank outlet pipe 23 and the return pipe 25 to cut off the external circulation circuit during maintenance or under specific operating conditions.

[0018] Inner loop components such as Figure 4 As shown, the internal circulation assembly 3 includes a multi-port pipe 31, branch pipes 32, an inlet pipe 33, a lower ring pipe 34, a heat-conducting pipe 35, a closed heat-conducting pipe 35a, an upper ring pipe 36, an outlet pipe 37, and a regulating pipe 38. The multi-port pipe 31 is fixedly installed on the inner side of the side wall 13, with its outer inlet connected to the inner end of the return pipe 25. Its inner end has multiple branch pipes 32, the number of which corresponds to the number of heat exchange units 5. Each branch pipe 32 is connected to the valve inlet 42 of a self-regulating valve assembly 4. The outer end of the inlet pipe 33 is connected to the valve outlet 43 of the self-regulating valve assembly 4, and its inner end extends downward and connects to the lower ring pipe 34. The lower ring pipe 34 is an annular pipe that surrounds the bottom outer side of the high-voltage coil 18 and is fixed to the high-voltage coil 18 by an insulating bracket. The upper ring pipe 36 is an annular pipe that surrounds the top outer side of the high-voltage coil 18 and is also fixed by an insulating bracket. Multiple heat-conducting pipes 35 are connected between the lower ring pipe 34 and the upper ring pipe 36. These heat-conducting pipes 35 are made of copper or stainless steel, with a diameter of 6-10 mm and a wall thickness of 1-1.5 mm. The heat-conducting pipes 35 penetrate the interior of the heat exchange unit 5, with 2-4 heat-conducting pipes 35 running through each heat exchange unit 5, at least one of which is a closed heat-conducting pipe 35a. The upper end of this closed heat-conducting pipe 35a is closed, while the lower end is open, with its lower end connected to a regulating pipe 38. The regulating pipe 38 is a thin-diameter capillary tube (inner diameter 2-4 mm), and its other end is connected to the oil return chamber 46 of the self-regulating valve assembly 4 (see...). Figure 5 The sealed heat pipe 35a is filled with transformer oil, forming a closed hydraulic conduction path.

[0019] The upper ring pipe 36 is connected to the outlet pipe 37. The inner end of the outlet pipe 37 is connected to the upper ring pipe 36, and its outer end is gathered and passes through the side wall 13 to connect to the outer oil tank inlet pipe 22.

[0020] heat exchange unit such as Figure 7 and Figure 8 As shown, the heat exchange unit 5 has a flat rectangular structure and is made of a metal material with good thermal conductivity (such as copper or aluminum alloy). The dimensions of each heat exchange unit 5 are designed according to the coil gap, with a typical thickness of 12-18 mm, a width of 80-120 mm, and a height of 150-250 mm. The heat exchange units 5 are fixed at equal intervals in the annular gap between the low-voltage coil 17 and the high-voltage coil 18, and their two side surfaces are in contact with or adjacent to the outer surface of the low-voltage coil 17 and the inner surface of the high-voltage coil 18, respectively, in order to efficiently conduct the heat generated by the coils.

[0021] like Figure 8 As shown in the partially enlarged structure, the heat exchange unit 5 includes a heat-conducting square tube 51, a honeycomb cavity 52, and an elastic membrane 53. The heat-conducting square tube 51 is a tubular shell with a rectangular cross-section and an internal cavity. The inner wall of the heat-conducting square tube 51 is provided with a honeycomb cavity 52, which is made of a porous heat-conducting material, such as expanded graphite foam, aluminum foam, or porous copper, with a porosity controlled between 60% and 70%. The pores of the honeycomb cavity 52 are filled with a phase change material. In this embodiment, a paraffin-based composite phase change material is used, with a phase change temperature set at 60-70℃ (selected according to the transformer insulation level), a latent heat of phase change ≥150kJ / kg, and a melting volume expansion rate of 10%-15%. The porous structure of the honeycomb cavity 52 forms capillary constraints on the phase change material, preventing liquid phase leakage, thus constituting a "shape-stabilized phase change material" structure.

[0022] An elastic membrane 53 is fixedly attached to the inner wall surface of the honeycomb cavity 52. ​​The elastic membrane 53 is made of an oil-resistant and highly elastic polymer material, such as fluororubber, nitrile rubber, or silicone rubber, with a membrane thickness of 0.2-0.3 mm. The elastic membrane 53 separates the phase change material inside the honeycomb cavity 52 from the through heat-conducting pipe 35, and can transfer the volume force generated by the expansion of the phase change material to the heat-conducting pipe 35. The heat-conducting pipe 35 and the closed heat-conducting pipe 35a pass through the heat exchange unit 5, and their pipe walls are in contact with or adjacent to the elastic membrane 53. When the phase change material expands due to heat, the pressure inside the honeycomb cavity 52 increases, pushing the elastic membrane 53 to bulge and deform in the direction of the heat-conducting pipe 35, thereby compressing the pipe wall of the heat-conducting pipe 35 (especially the closed heat-conducting pipe 35a).

[0023] Automatic control valve assembly, such as Figure 5As shown, the self-regulating valve assembly 4 includes a valve body 41, a valve core 44, and a spring component 47. The valve body 41 is made of aluminum alloy or stainless steel and has a cross-shaped internal cavity structure. The left end of the horizontal internal cavity is the valve inlet 42, and the right end is the valve outlet 43. The lower end of the vertical internal cavity is the return oil chamber 46, and the upper end is where the spring component 47 is installed. The valve inlet 42 is connected to the branch pipe 32, the valve outlet 43 is connected to the inlet pipe 33, and the return oil chamber 46 is connected to the regulating pipe 38. The valve core 44 is a cylinder made of nylon + glass fiber composite material or polytetrafluoroethylene, which has good self-lubricating properties and oil resistance. The valve core 44 is slidably fitted into the return oil chamber 46, with its lower end face forming a hydraulic action surface with the return oil chamber 46, and its upper end face abutting against the spring component 47. A flat hole 45 is radially penetrating through the middle of the valve core 44. The flat hole 45 is an elongated hole with its width gradually changing along the axial direction of the valve core 44. In this embodiment, the lower end of the flat orifice 45 is narrower (approximately 2 mm), while the upper end is wider (approximately 8 mm). This allows the cross-sectional area of ​​the flat orifice 45 and the transverse inner cavity of the valve body 41 to gradually increase as the valve core 44 moves upward, thereby achieving a linear adjustment relationship between the valve opening and the valve core displacement. The spring component 47 is installed at the upper end of the vertical inner cavity of the valve body 41, with its lower end contacting the upper surface of the valve core 44 and its upper end being pressed by the adjusting screw. The spring component 47 provides a downward elastic force, which balances the upward hydraulic thrust in the return oil chamber 46.

[0024] In a preferred embodiment of the invention, the spring component 47 is a shape memory alloy (SMA) spring, specifically made of nickel-titanium alloy wire wound and heat-treated for shaping. The SMA spring remains in an austenitic state at room temperature, exhibiting a high elastic modulus (stiffness K≈2N / mm). When electrically heated to the phase transformation temperature (approximately 70-80°C), the SMA spring transforms into a martensitic state, significantly reducing its elastic modulus (stiffness can drop below 0.5N / mm). Leads are connected to both ends of the SMA spring, and the current can be precisely controlled by a controller to dynamically adjust its elastic force.

[0025] The system also includes a controller (not shown in the figure) and multiple sensors. These sensors include a temperature sensor mounted on the surface of the heat exchange unit 5 to monitor the temperature of the phase change material; additional temperature sensors can be installed at locations such as the top oil temperature of the transformer and hot spots in the windings. A pressure sensor, optionally mounted in the return oil chamber 46 or the regulating pipe 38, is used to monitor hydraulic signals. An ultrasonic sensor, in a preferred embodiment, is embedded within the heat exchange unit 5. By transmitting and receiving coded ultrasonic pulses, it detects the solid-liquid interface position of the phase change material within the honeycomb cavity 52 in real time, thereby calculating the solid-liquid ratio α. The controller employs an industrial-grade embedded controller (such as an ARM Cortex-M4 or FPGA) to acquire signals from each sensor and run the thermal management control algorithm. The controller output is connected to the drive circuit of the SMA spring and the control terminal of the constant pressure pump 27.

[0026] like Figure 9 The working process and principle are as follows: 1. Natural cooling mode (normal load): When the transformer is under normal load (load rate ≤ 70%), the coil temperature is low, the phase change material in the heat exchange unit 5 remains solid, and there is no volume expansion. The elastic membrane 53 is in a relaxed state and has no squeezing effect on the closed heat conduction pipe 35a. The hydraulic pressure in the return oil chamber 46 is at normal pressure (balanced with the static pressure provided by the constant pressure tank 24), and the valve core 44 is in the lower position under the pressure of the spring component 47, and the opening of the flat hole 45 is at its minimum (about 10%-20%). At this time, the external circulation cooling device 2 is in standby or low flow state, and the system mainly relies on the original natural convection channel 6 of the transformer for cooling, and the heat sink 14 completes the basic heat dissipation.

[0027] 2. Physically self-driven cooling mode (overload condition): When the transformer encounters a short-term overload (such as peak charging at a charging station or starting up a data center server), and the load rate rises to 120%-180%, the heat generated by the coil increases sharply. The heat is rapidly conducted to the phase change material inside the honeycomb cavity 52 through the heat-conducting square tube 51. When the temperature of the phase change material reaches the phase change point (approximately 65°C), it begins to melt and expand in volume.

[0028] The volume force generated by the expansion of the phase change material acts on the elastic membrane 53, causing the elastic membrane 53 to bulge and deform towards the closed heat-conducting pipe 35a, compressing the pipe wall of the closed heat-conducting pipe 35a. Since the closed heat-conducting pipe 35a is filled with incompressible transformer oil, its internal pressure rises rapidly. This pressure is transmitted to the oil return chamber 46 of the self-control valve group 4 through the regulating pipe 38.

[0029] After the pressure in the return oil chamber 46 increases, an upward thrust is generated on the lower end face of the valve core 44. When this thrust exceeds the preload of the spring component 47, the valve core 44 begins to move upward. During the upward movement of the valve core 44, the cross-sectional area of ​​the flat orifice 45 and the transverse inner cavity of the valve body 41 gradually increases, the flow resistance from the valve inlet 42 to the valve outlet 43 decreases, and the flow rate of the self-control valve assembly 4 increases.

[0030] The constant-pressure cooling oil from the constant-pressure tank 24 enters the valve inlet 42 of the automatic control valve group 4 via the return pipe 25, multi-port pipe 31, and branch pipe 32. After passing through the flat orifice 45, it flows out from the valve outlet 43. Then, it enters the lower ring pipe 34 via the inlet pipe 33 and is distributed to each heat-conducting pipe 35. During the flow of the cooling oil through the heat-conducting pipe 35, it directly absorbs the heat transferred by the heat exchange unit 5 (including the heat conducted by the coil and the latent heat accumulated by the phase change material). After its temperature rises, it flows into the upper ring pipe 36. The hot oil flows into the outer oil tank 21 via the outlet pipe 37 and the outer oil tank inlet pipe 22.

[0031] The hot oil in the outer oil tank 21 dissipates heat to the environment through the independent heat sink 26. After the temperature drops, the oil settles to the bottom of the outer oil tank 21 and flows back to the constant pressure tank 24 through the outer oil tank outlet pipe 23, completing a complete external circulation.

[0032] In the above process, the opening degree of the self-controlled valve group 4 is entirely determined by the melting degree of the phase change material: the more severe the overload and the higher the melting ratio of the phase change material, the stronger the compression of the closed heat conduction pipe 35a, the higher the pressure of the return oil chamber 46, the larger the opening degree of the valve core 44, and the larger the external circulation flow. This constitutes a purely physical adaptive closed loop that requires no electronic control.

[0033] 3. Intelligent Correction Cooling Mode (Extreme Overload or Warning Conditions): When the load reaches the extreme overload (e.g., above 200%) or the phase change material is about to completely melt (solid-liquid ratio α ≥ 0.9), the physical self-driving mechanism gradually reaches its limit. At this time, if the same cooling intensity is maintained, it may not be able to meet the heat dissipation requirements.

[0034] In a preferred embodiment employing an SMA spring, the controller determines that the system has entered an extreme operating condition based on monitoring data from temperature and ultrasonic sensors. The controller calculates the required additional cooling advance and then outputs a predetermined pulse current (e.g., 0.1-0.3A, lasting 1-2 seconds) to the SMA spring.

[0035] When the SMA spring is energized, it generates Joule heat, raising its temperature above the phase transformation point and causing a martensitic phase transformation. This reduces its elastic modulus and stiffness. This means the downward pressure exerted by the spring component 47 on the upper end of the valve core 44 decreases. Under the same pressure in the return oil chamber 46, the valve core 44 can achieve a greater upward displacement, thereby further increasing the opening of the flat orifice 45 and correspondingly increasing the external circulation flow. For example, when the phase transformation material is completely melted (α=1), the pressure in the return oil chamber 46 reaches its peak value P_max. At this point, if the SMA spring maintains high stiffness, the valve core 44 opening can reach 80%; if the SMA spring is energized and softened, the valve core 44 opening can be increased to 100%, adding an additional 20%-30% cooling flow. This "intelligent correction" mechanism provides a safety margin for the system under extreme operating conditions.

[0036] 4. Recovery Mode: After the overload ends and the load returns to normal, the coil temperature drops, and the phase change material begins to solidify and shrink. The pressure exerted by the elastic membrane 53 on the sealed heat pipe 35a decreases, and the pressure in the oil return chamber 46 decreases. Under the action of the spring component 47, the valve core 44 gradually moves down and resets, the opening of the flat orifice 45 decreases, and the external circulation flow rate drops back to the standby state. At the same time, the SMA spring cools naturally after being de-energized, restoring its high stiffness state and preparing for the next overload.

[0037] To ensure the reliable operation of the system, the hydraulic transmission part needs to meet the following matching relationships: Let the maximum expansion pressure of the phase change material be P_pcm_max, the effective acting area of the elastic membrane 53 be A_m, and the effective extrusion area of the pressure-receiving section of the closed heat-conducting tube 35a be A_t. Then the maximum pressure P_c_max transmitted to the oil return chamber 46 can be approximately expressed as: P_c_max = (P_pcm_max × A_m × η) / A_c, where A_c is the effective area of the lower end face of the valve core 44, and η is the transmission efficiency (depending on the stiffness of the elastic membrane 53 and the tube wall). The pre-tightening force F_pre of the spring component 47 needs to meet: F_pre < P_c_min × A_c, where P_c_min is the minimum control pressure required for the valve core 44 to start moving (corresponding to the pressure when the phase change material starts to melt). The relationship between the spring stiffness K and the displacement x of the valve core 44 is: F_spring = F_pre + K·x. The relationship between the flow area S of the flat hole 45 and the displacement x of the valve core 44 is designed as a linear or upwardly convex curve to achieve an optimized match between the flow rate and the displacement. In this embodiment, a gradually expanding flat hole is adopted, so that the flow rate increases approximately linearly with the increase of the displacement.

[0038] In the above solution, the heat exchange unit can be made of a copper heat-conducting square tube 51 with dimensions of 100 mm × 15 mm × 200 mm and a wall thickness of 1.5 mm. The honeycomb cavity 52 uses expanded graphite foam with a porosity of 65% and a pore diameter of 2 - 3 mm. The phase change material is paraffin (model RT65), with a phase change temperature of 65 °C, a latent heat of 180 kJ / kg, and a volume expansion rate of 12%. The elastic membrane 53 uses a fluororubber membrane with a thickness of 0.25 mm and a Shore hardness of 60A. Two heat-conducting tubes 35 (Φ8 mm × 1 mm copper tubes) and one closed heat-conducting tube 35a penetrate through each heat exchange unit. The installation position is in the annular gap between the low-voltage coil 17 and the high-voltage coil 18, and 24 heat exchange units 5 (8 for each phase) are installed at equal intervals along the circumferential direction. A heat-conducting silicone grease is applied between the heat exchange unit 5 and the coil surface to reduce the contact thermal resistance.

[0039] The valve body 41 of the automatic control valve group is machined from 6061 aluminum alloy. The valve core 44 is made of polytetrafluoroethylene with a diameter of 20 mm. The flat hole 45 is a long waist hole (2 mm wide at the bottom, 8 mm wide at the top, and 12 mm long). The spring component 47 is tested using two schemes: a common stainless steel spring (control group) and a SMA spring (experimental group). The external oil tank 21 of the external circulation device has dimensions of 800 mm × 600 mm × 100 mm, and the independent radiator 26 is an aluminum fin (with a spacing of 10 mm and a height of 50 mm). The constant pressure tank 24 has a volume of 50 L, the constant pressure pump 27 has a power of 0.75 kW, and maintains a pressure of 0.1 ± 0.01 MPa.

[0040] Example 2: 2000kVA Data Center Dedicated Transformer. This example presents a completely new design for a dedicated transformer for a large data center. Data center power supply requires extremely high reliability, and servers generate a 300% instantaneous inrush current during startup, lasting 5-10 minutes. Compared to Example 1, the following optimizations can be made.

[0041] The heat exchange unit 5 and the coil undergo vacuum pressure impregnation treatment simultaneously. The heat-conducting square tube 51 and the coil wires use the same insulating enameling process, making the heat exchange unit 5 and the coil an integrated structure, reducing the contact thermal resistance by approximately 30%. Each heat exchange unit contains four heat-conducting tubes 35 (two of which are closed heat-conducting tubes 35a). The heat-conducting tubes 35 employ an internal finned tube structure, with microgrooves machined on the inner wall to enhance the heat transfer coefficient. The phase change material is a ternary eutectic mixture of fatty acids, with a phase change temperature of 70℃ and a latent heat of 200 kJ / kg.

[0042] Multi-level intelligent control strategy: The controller performs graded control of the SMA spring based on the solid-liquid ratio α fed back in real time by the ultrasonic sensor: α < 0.8: The SMA spring is not energized, maintaining high stiffness in a purely physical self-driven mode; 0.8 ≤ α < 0.95: A 0.1A current is applied to the SMA spring, reducing stiffness by 20% and increasing valve opening by 15%; α ≥ 0.95: A 0.25A current is applied to the SMA spring, reducing stiffness by 60% and increasing valve opening by 50%. Simultaneously, when an impact exceeding 300% is predicted, the SMA spring is preheated 30 seconds in advance, allowing the system to enter a high-flow state earlier. Digital twin interface: The controller reserves an Ethernet communication interface to receive load prediction data from the data center task scheduling system, enabling feedforward-feedback composite control.

[0043] The above embodiments and accompanying drawings are merely illustrative examples of the core principles and key structures of the system and method of the present invention. The accompanying drawings are simplified schematic diagrams, intended to clearly illustrate the structural, process, or data flow relationships related to the innovative points of the technical solution, and are not intended to limit the complete form of the actual product. This specification focuses on the innovative technical means necessary to achieve the purpose of the invention and solve the technical problem; auxiliary or common-sense details such as sealing structures, insulation treatments, standard component selection, and conventional filtering algorithms, which can be implemented by those skilled in the art without creative effort, are not described in detail, but should be understood as naturally included in the specific implementation of the present invention and fall within the protection and implementation scope of this technical solution.

Claims

1. A transformer adaptive internal and external dual-circulation thermal management system, comprising a transformer body (1), wherein the transformer body (1) is provided with an oil cavity (11), wherein the oil cavity (11) is provided with an iron core (16), a low-voltage coil (17) fitted outside the iron core (16), and a high-voltage coil (18) fitted outside the low-voltage coil (17), wherein an annular gap is provided between the low-voltage coil (17) and the high-voltage coil (18), and wherein the oil cavity (11) is provided with an original cooling channel (6) relying on the natural convection of oil, characterized in that, Also includes: An external circulation cooling device (2) is set outside the transformer body (1) to form an independent external circulation cooling circuit for oil; an internal circulation component (3) is set inside the oil chamber (11) and includes an inlet pipe, a collector pipe and multiple heat-conducting pipes (35) connected to the external circulation cooling device (2). Multiple phase change heat transfer modules (5) are disposed in the annular gap between the low-voltage coil (17) and the high-voltage coil (18). Each phase change heat transfer module (5) includes a heat-conducting shell (51) and a phase change material filled in the heat-conducting shell (51). The heat-conducting pipe (35) penetrates the interior of the heat-conducting shell (51) and is adjacent to the phase change material. A flow rate regulating unit is installed in the external circulation cooling circuit to regulate the circulation flow rate of the external circulation cooling device (2); a pressure transmission unit is connected between the phase change heat transfer module (5) and the flow rate regulating unit to transmit the pressure change generated by the thermal expansion of the phase change material to the flow rate regulating unit; wherein, when the phase change material is thermally expanded, the pressure transmission unit drives the flow rate regulating unit to increase the circulation flow rate of the external circulation cooling device (2).

2. The transformer adaptive internal and external dual-circulation thermal management system according to claim 1, characterized in that, The pressure transmission unit includes a closed hydraulic transmission pipe (38), one end of which is associated with an elastic pressure transmission element (53) in the phase change heat transfer module (5), and the other end is connected to a flow regulation unit.

3. The transformer adaptive internal and external dual-circulation thermal management system according to claim 2, characterized in that, The phase change heat transfer module (5) also includes an elastic pressure transmission element (53) disposed on the inner wall of the phase change material cavity (52), and one end of the hydraulic transmission pipe (38) is adjacent to or in contact with the elastic pressure transmission element (53).

4. The transformer adaptive internal and external dual-circulation thermal management system according to claim 1, characterized in that, The flow regulating unit is a hydraulically driven valve (4), including a valve body (41) and a valve core (44) slidably disposed in the valve body (41). The valve body (41) is provided with a valve inlet (42), a valve outlet (43) and a control chamber (46). The control chamber (46) is connected to the pressure transmission unit. The valve core (44) is provided with a flow passage (45) connecting the valve inlet (42) and the valve outlet (43). One end of the valve core (44) abuts against the elastic reset member (47).

5. The transformer adaptive internal and external dual-circulation thermal management system according to claim 4, characterized in that, The flow passage (45) is a flat hole composed of multiple parallel fine holes extending axially, and its width is gradually set along the moving direction of the valve core (44).

6. The transformer adaptive internal and external dual-circulation thermal management system according to claim 4, characterized in that, The elastic reset component (47) is a shape memory alloy spring. The system also includes a controller and a temperature sensor installed in the transformer body (1). The controller controls the energization and heating of the shape memory alloy spring according to the temperature signal detected by the temperature sensor, so as to dynamically adjust its elastic force.

7. The transformer adaptive internal and external dual-circulation thermal management system according to claim 6, characterized in that, The phase change heat transfer module (5) is equipped with an ultrasonic sensor for detecting the solid-liquid ratio of the phase change material. The controller controls the energization and heating of the shape memory alloy spring according to the solid-liquid ratio. When the solid-liquid ratio of the phase change material reaches a set threshold, the elastic force of the shape memory alloy spring is reduced, so that the valve core (44) can obtain a larger opening under the same control chamber pressure.

8. The transformer adaptive internal and external dual-circulation thermal management system according to claim 1, characterized in that, The external circulation cooling device (2) includes an external oil tank (21), an independent heat sink (26) fixed to the outside of the external oil tank (21), a constant pressure tank (24) and a constant pressure pump (27). The oil inlet of the external oil tank (21) is connected to the manifold, and the oil outlet is connected to the inlet of the constant pressure tank (24) through a pipeline. The outlet of the constant pressure tank (24) is connected to the inlet of the flow regulating unit through a return pipe (25).

9. A transformer thermal management control method based on the system according to any one of claims 1 to 8, characterized in that, Includes the following steps, Step S1: Under normal load conditions, the oil is cooled by the natural convection cooling channel (6) inside the transformer body (1) through the upper and lower convection. Step S2: When the load increases, causing the temperature between the low-voltage coil (17) and the high-voltage coil (18) to rise, the phase change material in the phase change heat exchange module (5) absorbs heat and expands. The pressure change generated by the expansion is transmitted to the flow regulation unit through the pressure transmission unit, which drives the flow regulation unit to increase the circulation flow of the external circulation cooling device (2), so that the cooling oil flows through the heat pipe (35) of the internal circulation component (3) through the phase change heat exchange module (5) for enhanced heat exchange. Step S3: When the load returns to normal and the coil temperature drops, the phase change material solidifies and shrinks, the pressure of the pressure transmission unit decreases, the flow regulation unit is reset, the circulation flow of the external circulation cooling device (2) decreases, and the system returns to step S1.

10. The transformer thermal management control method according to claim 9, characterized in that, The flow regulation unit includes a hydraulic drive valve (4), and the upper end of the valve core (44) of the hydraulic drive valve (4) is provided with a shape memory alloy spring (47). The method also includes an intelligent correction step: Step S4: The temperature of the phase change material or the solid-liquid ratio α is monitored in real time by a temperature sensor or ultrasonic sensor set in the phase change heat transfer module (5); Step S5: When it is detected that the phase change material is about to completely melt (α≥ set threshold) or when it is predicted that an extreme overload will occur, the controller energizes and heats the shape memory alloy spring (47) to reduce its elastic force, so that the valve core (44) can obtain a larger displacement under the same pressure transmission unit, further increasing the opening of the flow channel (45) and realizing early warning enhanced cooling under extreme working conditions.

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